As a technique for improving the moving picture display performance of a liquid crystal display device such as an LCD TV, a method of segmenting a liquid crystal backlight source into a plurality of blocks and controlling the timing of lighting for each segment block has been studied. For example, such a technique is disclosed in the following publication.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-99367
As illustrated in FIG. 1, the patent document 1 discloses a configuration in which driving circuits 28 to 31 independently drive backlights 32 to 35, which are segmented in four blocks, respectively.
As a technique for reducing the power consumption of a liquid crystal display device, a control method called as APL-AGC (Average Picture Level Automatic Gain Control) is known. In APL-AGC, the brightness of backlight is controlled depending on average video brightness. For example, such a technique is disclosed in the following publications.    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-156951    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2002-258401    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2002-357810    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2004-085961
The following methods are known as a preventive measure against signal overflow when one or more gains are applied to a video signal for, for example, securing sufficient lightness per power consumption or adjusting lightness.
FIG. 1 is a diagram that schematically illustrates a first example of a conventional preventive measure against overflow. As illustrated in FIG. 1(a), in this example, an overflow detection circuit 601 and a multiplexer 602 make up an overflow limiter, which clips an input value that is larger than the maximum output value (a region greater than 100% in the horizontal axis of FIG. 1(b)) into the maximum output value (100% in the vertical axis of FIG. 1(b)) independently for each of R, G, and B.
FIG. 2 is a diagram that schematically illustrates a second example of a conventional preventive measure against overflow. As illustrated in FIG. 2(a), in this example, gamma correction characteristics are applied to each of RGB using conversion tables 604a, 604b, 604c. As illustrated in FIG. 2(b), a change in output relative to a change in input gradually decreases as it approaches overflow. A non-linear gamma suppressor is configured so that, while maintaining monotonic increase continuously, it is ensured that the output level should fall within the maximum output range.
According to these methods, however, different gains are controlled independently for RGB. Except for white, which is a color in which the levels of RGB are equal to one another by nature, a reproduced color will not be faithful to the original color if any of RGB becomes saturated or if its gain decreases, that is color shift.
When the greatest level component of RGB becomes saturated first, a color shifts toward the remaining components. Then, when the second greatest level component becomes saturated, the color shifts toward white, that is, in the direction in which the color becomes faint.
FIG. 3 is a diagram that schematically illustrates an example of the disruption of a color balance that occurs when the non-linear gamma suppressor illustrated in FIG. 2 is employed. Flesh color having a color ratio of R:G:B=4:3:2 is taken as an example. As illustrated in the drawing, in a region where an input level is low, an output level increases while keeping the ratio of 4:3:2 (refer to points “a” and “b” in the drawing). As the input level comes close to 100% with increasing suppression, the R component becomes saturated first, which causes the color to shift toward yellow (refer to a point “c”). As lightness further increases, the G component becomes saturated next. As G becomes saturated, the color shifts toward white (refer to a point “d”). When a color shift toward white occurs in a flesh-color region, it means that the color shifts in a complementary-color direction for flesh color. As a result, the color does not look white but it looks to have a tinge of aqua blue, which results in a very unnatural picture.
Even when an original color is white, the values of RGB will be slightly different from one another due to white balancing and/or color temperature adjustment. In such a case, when a level enters a saturation region or other non-linear region, a white balance at a highlight peak part becomes disrupted. Even if a white balance is in an off-balance state as a whole in a certain direction, it is difficult for an observer to visually recognize the off-balance state unless a comparative screen picture is displayed adjacent to an off-balance picture at the same time because human eye adapts to such an off-balance state. However, when there are originally white regions whose lightness only is different from one another, such regions as shades of a snowy landscape or shades of a white shirt, even a slight disruption in a white balance makes these regions having different lightness conspicuously unnatural.
As described above, a means for preventing overflow could exert adverse effects on an image. However, to obtain lightness per power consumption as great as possible and reduce power consumption, there is a demand for a technique for effectively avoiding degradation in image quality even under considerable overflow and thereby offering more natural video display.